1. Field of the Invention
The present invention relates to lithium secondary batteries.
2. Description of Related Art
Lithium secondary batteries using a non-aqueous electrolyte and performing a charge-discharge operation by shifting lithium ions between positive and negative electrodes have been utilized in recent years as a new type of high power, high energy density secondary battery.
As for electrodes for such lithium secondary batteries, some research has been conducted on electrodes that use a material capable of alloying with lithium as its negative electrode active material. One example of the material capable of alloying with lithium that has been studied is silicon. However, a problem with a material capable of alloying with lithium such as silicon has been that the active material expands and shrinks when it intercalates (absorbs) and deintercalates (desorbs) lithium, causing the active material to pulverize or peel off from the current collector as the charge-discharge process is repeated. As a consequence, the current collection performance in the electrode reduces, degrading the battery's charge-discharge cycle performance.
The present applicant has found that an electrode formed by depositing on a current collector an active material thin film that intercalates and deintercalates lithium, such as an amorphous silicon thin film or a microcrystalline silicon thin film, shows high charge-discharge capacity and excellent charge-discharge cycle performance. (See International Publication WO 01/29913).
In this type of electrode, the active material thin film is divided into columnar structures by gaps formed along its thickness, and bottom portions of the columnar structures are in close contact with the current collector. In the electrode with such a structure, spaces form around the columnar structures. These spaces alleviate stress caused by the expansion and shrinkage of the thin film associated with charge-discharge cycles and prevent the occurrence of stress that causes the active material thin film to peel off from the current collector. Therefore, such an electrode can attain excellent charge-discharge cycle performance.
It is believed, however, that because of the electrode structure in which the negative electrode active material thin film is divided into columnar structures by gaps that form along its thickness, the above-noted electrode has a large surface area of the active material that comes into contact with the electrolyte solution, which can accelerate a decomposition reaction of the electrolyte solution. It is believed that, consequently, a larger amount of reaction product forms on the electrode surface than that formed on the surface of a negative electrode composed of a carbon material, which is commonly used at present, and the product diffuses through the electrolyte solution toward the positive electrode side and consequently promotes degradation of the positive electrode. As a result, due to the degradation of the positive electrode active material, the battery's charge-discharge cycle performance deteriorates.